Hydrogen discharge lamps, as referred to in the present application, are generally gas discharge lamps which are filled either with hydrogen or deuterium, or with a mixture of hydrogen and deuterium. A lamp of this type is described in U.S. Pat. No. 3,956,655, Pevo, the disclosure of which is hereby incorporated by reference.
Hydrogen discharge lamps lose radiation intensity during their lifetime, with respect to the initial radiation intensity available. Essentially, the loss in radiation emitted from the lamp is due to decrease of transmissivity of the bulb in which the electrodes and the fill are retained. The decrease in transmissivity is caused, primarily, by emitter material sputtered from the cathode, which may include barium, strontium, calcium and the like, which precipitates on the interior of the bulb. The material has a tendency to diffuse into the quartz glass and to react with the quartz. This interferes with the high transmissivity of pure quartz glass with respect to UV radiation, and particularly in the ranges of wave length below 250 nm. The radiation output thus decreases markedly over the lifetime of the lamp.
The lifetime of such a lamp is usually referred to as that time during which it can be operated, until the intensity of received radiation has dropped by about 50% or more, with respect to the initial radiation intensity. At a wave length of 230 nm, lamps with customary cathode construction then will have a lifetime of about 750 hours.
The referenced U.S. Pat. No. 3,956,655 describes a lamp having a fill of hydrogen or deuterium at low pressure. Rather than using quartz, the bulb is made of a boron silicate glass, e.g. of the Corning Type 9741 glass (tradename of Corning Glassworks). This increases the lifetime by about a factor of two in the wave length under question. The increased lifetime is believed to be due to the higher resistance of boron silicate glass with respect to alkaline earths than is the case with quartz glass. The lower inherent transmissivity regarding UV radiation with respect to quartz glass is compensated by utilizing a thinner wall. Thinning the walls of the bulb is obtained by fine drawing or tubular blowing of the bulb. This requires additional handling and, due to the reduction of wall thicknesses to only about 0.3 mm, renders the resulting bulb fragile and more subject to breakage.